Time-to-fire computer

ABSTRACT

1. In a gun fire control system for the interception of a target at a predetermined point in time prior to the time at which the target will arrive at the crossover point, apparatus comprising first means for generating a first signal proportional to target radial range, second means for generating a second signal proportional to target radial range rate, means coupled to said first and second means for multiplying said first and second signals and for producing a third signal having a first characteristic and having a value proportional to the product thereof, third means for generating a fourth signal proportional to the square of target absolute velocity, fourth means for generating a fifth signal proportional to the time of flight of a projectile to intercept said target, means coupled to said third and fourth means for multiplying said fourth and fifth signals to produce a sixth signal having a value proportional to the product thereof, multiplying means having two inputs one of which is adapted to receive said fourth signal and the other of which is adapted to receive a seventh signal, said seventh signal representing said predetermined time prior to the arrival of said target at said crossover point, said last means producing an eighth signal having a value proportional to the product of said fourth and seventh signals, means adapted to receive said sixth and eighth signals and operative to produce a ninth signal having a second characteristic and having a value proportional to the sum thereof, means for comparing the values of said third and ninth signals to produce a tenth signal having a value proportional to the difference therebetween and having the same characteristic as that one of said third and ninth signals having the greater value, and utilization means selectively responsive to a predetermined characteristic of said tenth signal, said tenth signal being applied to said utilization means.

United States Patent Saint Germain et all.

Apr. 17, 11973 [54] TIME-TO-FIRE COMPUTER [75] Inventors: Peter M. Saint Germain, Montclair, N.J.; Joseph F. Nekola, Huntington Station, NY.

[73] Assignee: Sperry Rand Corporation, New

York, NY.

[22] Filed: Dec. 6, 1956 [21] Appl. No.: 627,722

[52] U.S. Cl. ..235/6ll.5 E

[51] Int. Cl. ..G06g 7/80, 1P4lg 3/00 [58] Field of Search ..235/6l.5 E, 61.5 S,

[56] References Cited UNITED STATES PATENTS 3,063,047 ll/l962 Steele 235/615 E Primary Examiner-Samuel Feinberg Assistant Examinerl-l. A. Birmiel Attorney-Reginald V. Craddock EXEMPLARY CLAIM l. in a gun fire control system for the interception of a target at a predetermined point in time prior to the time at which the target will arrive at the crossover point, apparatus comprising first means for generating a first signal proportional to target radial range, second means for generating a second signal proportional to target radial range rate, means coupled to said first and second means for multiplying said first and second signals and for producing a third signal having a first characteristic and having a value proportional to the product thereof, third means for generating a fourth signal proportional to the square of target absolute velocity, fourth means for generating a fifth signal proportional to the time of flight of a projectile to intercept said target, means coupled to said third and fourth means for multiplying said fourth and fifth signals to produce a sixth signal having a value proportional to the product thereof, multiplying means having two inputs one of which is adapted to receive said fourth signal and the other of which is adapted to receive a seventh signal, said seventh signal representing said predetermined time prior to the arrival of said target at said crossover point, said last means producing an eighth signal having a value proportional to the product of said fourth and seventh signals, means adapted to receive said sixth and eighth signals and operative to produce a ninth signal having a second characteristic and having a value proportional to the sum thereof, means for comparing the values of said third and ninth signals to produce a tenth signal having a value proportional to the difference therebetween and having the same characteristic as that one of said third and ninth signals having the greater value, and utilization means selectively responsive to a predetermined characteristic of said tenth signal, said tenth signal being applied to said utilization means.

9 Claims, 3 Drawing Figures PATENTED APR '1 71975 PCT FCT

- FIRE CONTROL "COMPUTER INVENTORS PETER M. SAINT GERMAIN JOSEPH F. NEfflA BY '1 dM ATTORNEY TlME-TO-FIRE COMPUTER The invention herein described was made in the course of or under a contract or subcontract thereunder, with the Department of the Army.

The invention concerns gun director systems, and more particularly, relates to an improved time-to-fire computer for a gun fire control system.

Gun fire control systems are known in the prior art for computing data relating to target slant range, slant range rate, target velocity, and time of flight of a projectile to intercept the target as it moves along its path of travel. From a purely ballisticspoint of view, the optimum point of projectile interception of the target occurs when the target is located at the shortest distance from the gun along the path of its travel. This point is commonly termed the target cross-over point". Provision has been made in the prior art gun fire control systems for engaging the target at a predetermined point along its path, for example, the cross-over point. The future arrival of the target at the predetermined point is computed in the prior art systems with the aid of the aforementioned data.

Considerations other than those which are purely ballistic, however, influence the overall probability of the successful interception of the target. Such other considerations relate to the response of the gun fire control system employed and include: 1) whether the type of course the target is travelling is compatible with the corresponding assumptions made in the design of the computer; (2) whether the target is within the range of the gun fire control computer; and (3) whether the gun fire control computer has settled out with minimum residual error.

Experience with existing gun fire control systems has a indicated that particularly with the engagement of high velocity targets at relatively short cross-over ranges, excessive dynamic lag errors occur in the computer as the target approaches the cross-over point. Therefore, it is desirable that the target he engaged at some point along its path of travel prior to the cross-over point but after the aforementioned three considerations have been satisfactorily met. In order to avoid the dynamic lag problem of the computer it is further desirable that the target be engaged at an increasing distance before the cross-over point as the target velocity increases.

Upon the initial satisfaction of the above three considerations, firing may be commenced with a reasonable probability that the target will be successfully intercepted. However, if the assumption is made that the target will continue along its present course, there is a later and better time to commence firing with regard to hit probability. This later time should be as close as possible to the arrival of the target at the cross-over point but not so close to the cross-over point that the aforementioned excessive dynamic lags begin to occur in the gun fire computer. This is especially true if the number of rounds to be fired during a particular engagement is limited.

It is the general object of the present invention to avoid and overcome the foregoing disadvantages of prior art gun fire control systems by the provision of an improved time-to-fire computer.

Another object is to produce a time-to-fire signal for the interception of a target at'a point along its path of travel which is displaced a distance from the cross-over point as a function of target velocity.

An additional object of the invention is to provide means to produce a time-to-fire signal in a gun fire control system for the interception of a target at a predetermined point in time prior to the time at which the target will arrive at the cross-over point.

The foregoing and other objects, as will appear as the specification proceeds, are accomplished by a time-tofire computer adapted to receive target slant range, target velocity, and time of projectile flight information from an existing gun fire control system computer. Means are provided for multiplying the slant range by range rate, for multiplying an adjustable but predetermined time-to-fire by the square of target velocity, for multiplying the time of projectile flight by the square of target velocity and for summing the three products of multiplication.

in a preferred embodiment, the first product is represented by a voltage of first polarity while the second and third products are represented by a voltage of opposite polarity. When the first product is equal to the sum of the second and third products, the sum of all three products is equal to zero. The zero voltage null thus produced is employed to actuate a time-to-fire indicator.

For a more complete understanding of the present invention, reference should be had to the following description and the appended drawings of which:

FIG. 11 shows geometrical figures useful in developing the equations employed in the present invention, and

MG. 2 is a preferred embodiment of the present invention.

ln lFlG. la, the straight line locus of points A, B, T, and C define a portion of the targets path of travel whereon the target has uniform velocity. Point C represents the targets closest approach to gun G, said point being defined as the cross-over point. The total range of the targets path of interest, namely, between points A and C is sub-divided in time by points B and T. The distance AC is representative of the time necessary for the target to travel between points A and C, this time being termed present course time (PCT). The line AB represents the time of flight (i of a projectile from gun G to intercept the target at point B, assuming that the gun had been fired when the target is at point A and further assuming that the target is travelling from point A toward point C.

The time involved for the target to travel from point B to the cross-over point C is termed future course time" (PCT). The line TC represents a predetermined time (TF) prior to the arrival of the target at cross-over point C when gun G is to commence firing; the line ET represents the time to go t,,.

The various times illustrated in MG. lla may be related by the following equations:

FCT= Pcr- 2,, 1,

t FCT- TF 2 MG. llb shows the same geometrical relationship between flight path AC traveled by the target and the position of gun G. in the case of FlG. lb, however, velocity magnitudes, rather than time intervals, are superimposed on the figure. The distance between points A and 15 represents the velocity (V) of a target travelling along the path AC. The distance between points A and F represents the component of said velocity (D) existing along the radial line AG between the gun and the target when at position A. The line AC is equal to the product of present course time and velocity (PCT) V of the target while the line AG equals in length the radial distance (D) of the target from gun G when the target is located at A.

g The quantities shown in H6. 111) may be expressed by the following relationship:

whence Combining equations 1 and 4, there results Combining equations 2 and 5, there results z,,= (DD/V i, TF

z,,V =DDV (t,,+TF) (6) From FIG. lla, it can be seen that t,,= when PCT TF. It is at this time that the gun is to be tired. V being a finite quantity other than zero, the product t V must go to zero only when t, 0. Under these conditions, when t 0, then DD=V (t,,+TF) 1 The time-to-fire signal is produced by the apparatus of FIG. 2 when the condition t 0 is satisfied, namely, upon the occurrence of the relationship specified by equation 7.

In FIG. 2, a tire control computer, generally designated by the numeral 1, produces, for purposes of the present invention, five mechanical outputs designated by shafts 62, 63, 64, 113 and 6. These are not specifically required in mechanical form for the operation of the present invention but are shown in the preferred embodiment of FIG. 2 for the reason that said mechanical outputs are conveniently available from a prior art gun directing system such as disclosed in U.S. Pat. No. 2,660,371, issued on Nov. 24, 1953, in the names ofD. J. Campbell, et al., and assigned to the present assignee.

Briefly stated, the gun directing system of the aforesaid issued patent comprises a radar transmitter, receiver, and computer, the radar system proper producing target slant range, target azimuth, and target elevation information. The computer portion of the prior art gun directing system is adapted to receive said information and to compute therefrom the rectilinear components of target velocity and the time of projectile travel from a gun to the target as the target moves along its path of travel. The operation of the prior art gun directing system, per se, does not form a part of the present time-to-fire computer invention and for this reason is not described in detail here.

The output shafts 62, 63, 64, 113, and 6 of fire control computer i bear the same numerical designations as employed in the aforesaid U.S. Pat. No; 2,660,371. Shafts 62, 63, and 64 have displacements respectively proportional to the x, y, and h components of target velocity. Shaft 1t? has a displacement proportional to the aforementioned time of projectile flight while the displacement of shaft 6 is proportional to target slant range. Shafts 62, and 64 respectively position the sliders 2, 3, and 4 of potentiometers 5, 6, and 7. Potentiometers 5, 6, and 7 are center-tapped to ground as indicated. A source S of positive D- potential, relative to ground, is connected to both ends of potentiometers 5, 6, and 7.

inasmuch as the x, y, and h components of target velocity may individually increase or decrease relative the origin of the x, y, it coordinate system with the ap proach of an incoming target, the center-tapped arrangement of potentiometers 5, 6, and '7 operates to produce, at the respective sliders 2, 3, and 4 thereof, a D-C potential whose amplitude is proportional to the absolute magnitude of the respective velocity components irrespective of whether said velocity components are of a first sense (indicating positive 12,3}, and 1 1) or of the opposite sense (indicating negative 1'6, 3}, and h).

Potentiometers 5, 6, and 7 are wound so that the voltage appearing at respective sliders 2, 3, and 4 are proportional to the square of the displacements thereof. Accordingly, a voltage proportional to 5: is produced at slider 2, a voltage proportional to y is produced at slider 3, while a voltage proportional to h is produced at slider 4. The F, and 7 2 voltages are summed in a conventional resistive addition network comprising resistors 9, ill, and 1111. Thus there is produced on line 112 a voltage whose magnitude is proportional to the square of the target velocity, namely, to the sum of the squares of the component velocities (i j/ +11 The potential present on line 112, relative to ground, causes a flow of current through ammeter 113 and paralleled potentiometers 114 and 15 such that the deflection of ammeter i3 is proportional to the square of the target velocity. With appropriate calibration of amrneter 113, true target velocity may be indicated for the convenience of the time-to-fire computer operator.

The slider 116 of potentiometer 114 is driven by manually positioned raclt-and-pinion 1 7, the displacement of slider 16 along potentiometer 114 being calibrated in terms of time-to-iire (TF) previously defined in connection with FllG. ll. As is well known in the art, a potentiometer will effectively multiply the voltage impressed across it by a factor proportional to the displacement of the slider. Therefore, there appears at slider 116 a voltage whose amplitude is proportional to the product of target velocity squared and time-tofire (V TF).

The slider 18 of potentiometer 16 is directly driven by output shaft 113 of computer ll. As previously mentioned, the displacement of shaft 113 is proportional to the time of projectile flight (t also previously described in connection with FIG. ll. in a manner similar to the operation of multiplying potentiometer l4, there appears on slider iii of potentiometer 15 a voltage whose amplitude is proportional to the product of target velocity squared and time of projectile flight (17%.

The output shaft 6 of computer 11 assumes a displacement proportional to the radial range of the target. Shaft 6 is coupled to D-C generator 119 via step-up gear train 20. The purpose of gear train 2t?) will be more fully described later. Shaft 6 is connected to vary the position of slider 22 of potentiometer 23. The output of D- C generator 19 is impressed across the terminals of potentiometer 23. Inasmuch as the output of D-C generator 19 is proportional to the rate of change of target slant range (1 and since slider 22 is positioned according to the slant range of the target as derived from shaft 6, a voltage is produced at slider 22 of potentiometer 23 whose amplitude is proportional to the product of target slant range and target slant range rate (DD).

In the preferred embodiment of H6. 2, D-C generator 19 is so coupled to shaft 6 of computer ll that D-C generator 19 produces a negative voltage, with respect to ground, for approaching target (targets having a decreasing slant range). It should be noted that battery 8 is poled so that positive terminal thereof is connected to potentiometers 5, 6, and 7. The only fundamental requirement of the present invention is that the D-C generator 19 produce an output voltage of opposite polarity to that produced by D-C source 8 for incoming targets.

The voltages appearing on sliders l6, l8, and 22 are summed in conventional resistive summation network 24, 25, and 26 to produce on line 27 a D-C voltage whose amplitude may be expressed by the following equation:

E =DD-V (t,,-i-TF) (8) In the case of incoming targets, E will initially assume a relatively high nega tive potential. This can be seen upon considering the fact that the slant range rate of an approaching target is initially high and falls toward zero as the target approaches the cross-over point, at which point its radial velocity is zero. In view of the high initial radial velocity of the target, the quantity DD will be relatively large for distantly approaching targets.

Upon inspection of the other quantity shown in equation 8, namely V t, TF), it will be observed that the factor TF is a constant which was manually inserted into the apparatus on FIG. 2 by rack-and-pinion 117. The factor t,,, representing time of flight of a shell from the gun to the target will decrease as the target approaches the cross-over point. The remaining factor V was assumed constant in the derivation of equation 6. Therefore, the quantity DD will decrease more rapidly for an approaching target than will the quantity V (t, TF).

By comparing equations 6 and 8, it can be seen that E t, V According to the previous development of equation 6, the time-to-fire indicator is to be energized when t,= 0, or, in other words, when E 0.

Lamp 28 generally represents the time-to-fire indicator of the present invention. A suitable source of potential 29 is connected to one side of indicator lamp 28 while the other side thereof is returned to potential source 29 by means of closed contacts 3d and 311 of polarized relay 32. Polarized relay 32 is adapted to respond only to voltages appearing on line 27 which are positive with respect to ground. For all other voltages,

relay 32 is deenergized and contacts 361? and 3B are opened. Thus, as E appearing on line 27, decreases from its initial high negative value toward zero as the target approaches the cross-over point, relay 32 will remain deenergized and contacts 3d, 31 will remain open. At some point in time, prior to the arrival of the target at the cross-over point and displaced therefrom in time by an amount corresponding to the setting of the time-to-fire slider T6 of potentiometer 114, E will invert in polarity and become positive with respect to ground. At this point, relay 32 will be energized causing the closing of contacts 30 and 31, in turn producing illumination of lamp 2%. Upon the illumination of lamp 28, the time-to-fire computer operator commences firing at the approaching target.

The purpose of step-up gear train 24 will now be more fully described. By inspection of equation 6, it can be seen that the factor V with which 13 is multiplied, may be interpreted as representing a sensitivity factor in the operation of the time-to-fire computer apparatus of H6. 2. That is, for relatively large values of constant target velocity, the slope is quite high with which E appearing on line 27, varies from its high initial negative value through 0 to the necessary positive potential to energize relay 32. Conversely, the slope of E becomes less as the target approaches on a given course with a decreased constant velocity. As is well known in the art, relay 32 will have an inherent threshold of voltage levels to which the relay will respond. it can be appreciated, therefore, that the time interval during which relay 32 will respond to a voltage going through 0 in a positive direction will depend on the rate at which said voltage increases positively. In other words, the response of relay 32 to E will be more rapid for relatively high constant velocity targets than is the case with relatively low constant velocity targets travelling the same course. The time lag in the response of relay 32 for low constant velocity targets is clearly objectionable for purposes of indicating precisely the optimum time of firing on the approaching target, which is the objective of the apparatus of FIG. 2.

in order to shorten the time of response of relay 32, despite its inherent threshold lag effect, step-up gear train 26;) may be provided, for example, to increase the voltage output from D-C generator 19, for a given target slant range velocity. From another point of view, the range of output voltage values of D-C generator 119 is increased by means of gear train 2% (than would be the case were shaft 6 directly connected to generator l9) which necessarily produces a greater rate of change of output voltage for a given change of speed of rotation of shaft 6. Therefore, the rate is increased with which E goes through 0 in a positive direction for all values of target velocity. The result is that relay 32 will respond with a decreased time lag proportional to the gear ratio employed in train 20, the lag being reducible to an acceptable minimum for the lowest target velocity by merely increasing the step-up gear ratio of gear 2th. it is understood, of course, that the voltage level of D-C source b will have to be chosen, relative to the output voltage of D-C generator 119, so that equation 8 is satisfied.

it can be seen from the preceding description that the objects of the present invention have been achieved by the provision of a time-to-fire computer adapted to receive target velocity, target slant range, and time of projectile flight information from an existing fire control computer. The time-to-fire computer generates target slant range rate and multiplies it with target slant range to produce a D-C voltage of a first polarity proportional to the product thereof. The time-to-fire computer further operates to multiply a selectable time-tofire by the square of target velocity, to multiply the time of projectile flight by the square of target velocity and to sum the two products produced thereby to generate a D-C voltage of an opposite polarity whose amplitude is proportional to the sum of the products.

The first and second D-C voltages are compared and the difference voltage of predetermined polarity operates an indicator for purposes of producing an alert signal for the gun directing system operator to commence firing at a target.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. In a gun fire control system for the interception of a target at a predetermined point in time prior to the time at which the target will arrive at the crossover point, apparatus comprising first means for generating a first signal proportional to target radial range, second means for generating a second signal proportional to target radial range rate, means coupled to said first and second means for multiplying said first and second signals and for producing a third signal having a first characteristic and having a value proportional to the product thereof, third means for generating a fourth signal proportional to the square of target absolute velocity, fourth means for generating a fifth signal proportional to the time of fiight of a projectile to intercept said target, means coupled to said third and fourth means for multiplying said fourth and fifth signals to produce a sixth signal having a value proportional to the product thereof, multiplying means having two inputs one of which is adapted to receive said fourth signal and the other of which is adapted to receive a seventh signal, said seventh signal representing said predetermined time prior to thearrival of said target at said crossover point, said last means producing an eighth signal having a value proportional to the product of said fourth and seventh signals, means adapted to receive said sixth and eighth signals and operative to produce a ninth signal having a second characteristic and having a value proportional to the sum thereof, means for comparing the values of said third and ninth signals to produce a tenth signal having a value proportional to the difference therebetween and having the same characteristic as that one of said third and ninth signals having the greater value and utilization means selectively responsive to a predetermined characteristic of said tenth signal, said tenth signal being appiied to said utilization means.

2. Apparatus as defined in claim il wherein said means for multiplying and said multiplying means are eaghapotentiometer. I

0. Apparatus as denned in claim It wherein said second means comprises a differentiating means coupled to said first means and responsive to said first signal.

4. Apparatus as defined in claim 3 wherein said second means is coupled to said first means by a gain producing device.

in a gun fire control system for the interception of a target at a predetermined point in time prior to the time at which the target will arrive at the crossover point, apparatus comprising first means for generating a first signal proportional to target radial range, second means for generating a second signal proportional to target radial range rate, means coupled to said first and second means for multiplying said first and second signals and for producing a first voltage of a first polarity having an amplitude proportional to the product thereof, third means for generating a third signal proportional to the square of targets absolute voltage, fourth means for generating a fourth signal proportional to the time of flight of a projectile to intercept said target, means coupled to said third and fourth means for multiplying said third and fourth signals to produce a fifth signal having a value proportional to the product thereof, multiplying means having two inputs one of which is adapted to receive said third signal and the other of which is adapted to receive a sixth signal, said sixth signal representing said predetermined time prior to the arrival of said target at said crossover point,

said last means producing a seventh signal having a value proportional to the product of said third and sixth signals, means adapted to receive said fifth and seventh signals and operative to produce a second voltage of a second polarity having an amplitude proportional to the sum thereof, means adapted to receive said first and second voltages for producing a third voltage having an amplitude and polarity indicative of the algebraic summation thereof, and utilization means selectively responsive to a predetermined polarity of said third voltage, said third voltage being applied to utilization means.

6. Apparatus as defined in claim 5 wherein said switching means is a polarized relay.

'7'. Apparatus as defined in claim 5 wherein said second means comprises a differentiating means coupled to said first means and responsive to said first signal.

8. Apparatus as defined in claim 7 wherein said second means is coupled to said first means by a gain producing device.

9. Apparatus as defined in claim 5 wherein said utilization means comprises an indicating device, a source of energizing potential, and switching means responsive to a predetermined polarity of said third voltage for connecting said source of energizing potential to said indicating device. 

1. In a gun fire control system for the interception of a target at a predetermined point in time prior to the time at which the target will arrive at the crossover point, apparatus comprising first means for generating a first signal proportional to target radial range, second means for generating a second signal proportional to target radial range rate, means coupled to said first and second means for multiplying said first and second signals and for producing a third signal having a first characteristic and having a value proportional to the product thereof, third means for generating a fourth signal proportional to the square of target absolute velocity, fourth means for generating a fifth signal proportional to the time of flight of a projectile to intercept said target, means coupled to said third and fourth means for multiplying said fourth and fifth signals to produce a sixth signal having a value proportional to the product thereof, multiplying means having two inputs one of which is adapted to receive said fourth signal and the other of which is adapted to receive a seventh signal, said seventh signal representing said predetermined time prior to the arrival of said target at said crossover point, said last means producing an eighth signal having a value proportional to the product of said fourth and seventh signals, means adapted to receive said sixth and eighth signals and operative to produce a ninth signal having a second characteristic and having a value proportional to the sum thereof, means for comparing the values of said third and ninth signals to produce a tenth signal having a value proportional to the difference therebetween and having the same characteristic as that one of said third and ninth signals having the greater value and utilization means selectively responsive to a predetermined characteristic of said tenth signal, said tenth signal being applied to said utilization means.
 2. Apparatus as defined in claim 1 wherein said means for multiplying and said multiplying means are each a potentiometer.
 3. Apparatus as defined in claim 1 wherein said second means comprises a differentiating means coupled to said first means and responsive to said first signal.
 4. Apparatus as defined in claim 3 wherein said second means is coupled to said first means by a gain producing device.
 5. In a gun fire control system for the interception of a target at a predetermined point in time prior to the time at which the target will arrive at the crossover point, apparatus comprising first means for generating a first signal proportional to target radial range, second means for generating a second signal proportional to target radial range rate, means coupled to said first and second means for multiplying said first and second signals and for producing a first voltage of a first polarity having an amplitude proportional to the product thereof, third means for generating a third signal proportional to the square of target''s absolute voltage, fourth means for generating a fourth signal proportional to the time of flight of a projectile to intercept said target, means coupled to said third and fourth means for multiplying said third and fourth signals to produce a fifth signal having a value proportional to the product thereof, multiplying means having two inputs one of which is adapted to receive said third signal and the other of which is adapted to receive a sixth signal, said sixth signal representing said predetermined time prior to the arrival of said target at said crossover point, said last means producing a seventh signal having a value proportional to the product of said third and sixth signals, means adapted to receive said fifth and seventh signals and operative to produce a second voltage of a second polarity having an amplitude proportional to the sum thereof, means adapted to receive said first and second voltages for producing a third voltage having an amplitude and polarity indicative of the algebraic summation thereof, and utilization means selectively responsive to a predetermined polarity of said third voltage, said third voltage being applied to utilization means.
 6. Apparatus as defined in claim 5 wherein said switching means is a polarized relay.
 7. Apparatus as defined in claim 5 wherein said second means comprises a differentiating means coupled to said first means and responsive to said first signal.
 8. Apparatus as defined in claim 7 wherein said second means is coupled to said first means by a gain producing device.
 9. Apparatus as defined in claim 5 wherein said utilization means comprises an indicating device, a source of energizing potential, and switching means responsive to a predetermined polarity of said third voltage for connecting said source of energizing potential to said indicating device. 